Defence experts on Friday said that India should seek assistance from Russia while they undertake the rescue operation to locate and evacuate sailors on the burnt submarine, INS Sindhurakshak.

Indian navy divers have extracted four bodies from the wreckage of the burnt submarine.

Eighteen sailors were missing after weapons stored in the forward section of the Russian-built INS Sindhurakshak caught fire, after three explosions, and sank at the naval dockyard in Mumbai around midnight between August 13 and August 14.

India's navy on Friday said that divers have found the bodies of four sailors who were on board and added that finding any surviving personnel within the submarine is unlikely.

Former Lieutenant General (Retired) P.N. Hoon did not rule out the possibility of sabotage as the infiltration is now not only limited to men and houses but it has reached the forces as well.

He added that if it's an accident then it's a mistake from the side of Indian navy and they could be questioned as to how such an accident took place despite the presence of such trained members.

"We are not trained to get into the submarine. We don't know how to blast it, how to go in. I feel that it's best to ask for the Russian help, there is no harm in it, and go ahead," said Hoon.

According to a defence source the navy did not have a deep submergence rescue vehicle that other navies use to save trapped sailors, although in this case the incident occurred while it was docked and not in the deep seas.

The navy also said that the divers couldn't enter the Sindhurakshak for more than 12 hours because of boiling water inside parts of the vessel.

Access was "almost impossible due to jammed doors and hatches, distorted ladders, oily and muddy waters".

Only one diver could work at a time initially to clear a path inside the submarine. Divers are trying to reach further inside to find the remaining bodies, the navy said.

A naval board of inquiry has been ordered into how weapons went off while the vessel was berthed in the high-security Mumbai base.

Uday Bhaskar a defence expert said that seeing the complexity of the situation, it was difficult to comment on the chances of survival of the trapped sailors.

"It is evident that the front part of the submarine, the first two or three compartments, I am conjecturing, have been so badly damaged that they are not yet able to access them. So that is the reason why this is taking time because this is both tragic and a very-very complex unprecedented kind of accident. The divers are trying their very best and I think given the complexity and the damage sustained by the submarine, this may take a little longer," he said.

He added that although the probability of sabotage is low but nothing could be said before the probe in the incident was completed.

Armed with its latest multi-role missile system, radar and electronics, the submarine was to be the backbone of the Indian Navy.

The submarine was fitted with the Club-S multi-role missile system capable of eliminating targets at a distance of over 250 kilometres.

The submarine was under consideration to be equipped with the Brahmos cruise missiles.

INS Sindhurakshak, a Type 877EKM in Russia, was constructed at St. Petersburg in 1997.

The submarine was designed to patrol and to protect naval communications, assault warships, enemy submarines, land targets and perform naval reconnaissance.

It is one of the quietest diesel-electric submarines in the world mainly intended for anti-ship and anti-submarine operations.

10 Kilo-class submarines were constructed in Russia's shipyards for the Indian Navy from 1985-2000.